High dielectric strength member

ABSTRACT

A sprayed member is obtained by plasma spraying an oxide containing a rare earth element having atomic number 64 to 71 onto a substrate to form a spray coating. The sprayed member exhibits a high dielectric strength without a need for sealing treatment and is useful as dielectric rolls, heating substrates, electrostatic chucks, susceptors and the like.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to members having high dielectric strength spray coatings for use as sprayed members required to have a high dielectric strength, such as insulating coated members, spray coated heaters, semiconductor manufacturing susceptors and electrostatic chucks.

[0003] 2. Background Art

[0004] Conventional insulating ceramic coated members relying on the thermal spraying process include dielectric rolls for corona discharge treatment, heating substrates and electrostatic chucks for semiconductor manufacturing apparatus.

[0005] For example, dielectric rolls for corona discharge treatment are required to have a dielectric strength of at least 5 kV when ceramic coatings have a thickness of at least 300 μm. While currently available alumina sprayed coatings have a dielectric strength of approximately 10 kV/mm, ceramic coatings are made as thick as 500 μm to 3 mm in order to clear the requirement. Such thick ceramic coatings tend to craze or separate from supports. See JP-A 11-279302.

[0006] Also used as heating substrates are alumina spray coated members. They fail to maintain dielectric strength if the thickness of sprayed coating is less than 100 μm, and are prone to crack if the thickness of sprayed coating is more than 500 μm. Then sprayed coatings desirably have a thickness in the range of 100 to 500 μm. To enhance dielectric strength, pores in sprayed coatings must be sealed (see JP-A 2002-289329).

[0007] The processes of manufacturing semiconductor wafers and flat panel display substrates involve many substrate-processing steps like etching, deposition and exposure, for which electrostatic chucks, heaters, susceptors and the like are used in the processing chamber. In these steps, workpieces are often treated with a plasma of corrosive halide gas. Since those members serving as processing jigs in such an environment are attacked by corrosive species, use is typically made of ceramic material members and metal material members having ceramics sprayed thereon. Currently, ceramic sprayed members are often used because wafers become of a larger size and complex members combined with metal members such as heaters can be easily fabricated and because of a low cost.

[0008] Typical of ceramic sprayed members are alumina sprayed members. They are used as electrostatic chucks or the like. Because of their properties, however, alumina sprayed members need sealing treatment in order to provide a high dielectric strength. Organic materials are used in the sealing treatment. Such organic fills are susceptible to etching in a halogen plasma environment, becoming a cause of generating particles.

[0009] Since the recent halogen plasma process has a high selectivity and uses a high density plasma in order to form narrow and deep channels by etching, there arises a problem that even the alumina sprayed members are less resistant to the halogen plasma.

[0010] Attention is now drawn to Group IIIa compounds as the material having improved erosion resistance in a halogen plasma environment. Of these compounds, yttrium-containing oxides and fluorides are known to be resistant to halogen plasma erosion. Members having such oxides and fluorides sprayed thereon are disclosed in JP-A 2001-164354 and JP-A 2001-226773. However, sprayed coatings of alumina and yttria are still insufficient in dielectric strength, and must be made thick or subjected to sealing treatment.

SUMMARY OF THE INVENTION

[0011] An object of the invention is to provide a high dielectric strength member bearing a sprayed coating which has halogen plasma resistance and improved dielectric strength properties.

[0012] It has been found that a member having a sprayed coating of an oxide of an atomic number 64 to 71 rare earth element formed on a substrate exhibits a high dielectric strength without a need for sealing treatment on the sprayed coating and possesses halogen plasma resistance.

[0013] Accordingly, the present invention provides a high dielectric strength member comprising a substrate and a high dielectric strength coating formed thereon in the form of a sprayed coating of an oxide containing a rare earth element having atomic number 64 to 71.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The high dielectric strength member of the invention is arrived at by forming on a substrate a sprayed coating of an oxide containing a rare earth element having atomic number 64 to 71. The sprayed coating has a high dielectric strength without a need for sealing treatment.

[0015] The substrate may be selected from among ceramics, metals and composites thereof depending on a particular application, though not critical. Exemplary ceramic materials include shaped bodies composed mainly of quartz, alumina, magnesia and yttria, and complex oxides thereof, shaped bodies composed mainly of silicon nitride, aluminum nitride and boron nitride, and shaped bodies composed mainly of silicon carbide and boron carbide. Exemplary carbon materials include carbon fibers and sintered carbon bodies. Exemplary metal materials include those based on iron, aluminum, magnesium, copper, silicon and nickel, alloys thereof, for example, stainless alloys, aluminum alloys, anodized aluminum alloys, magnesium alloys and copper alloys, and single crystal silicon. Also included in the composite category are metal materials covered with ceramic coatings and aluminum alloys subjected to anodizing treatment or surface treatment, typically plating.

[0016] The sprayed coating contains an oxide of a rare earth element having atomic number 64 to 71, i.e., Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. It is most preferred that the sprayed coating consist solely of the rare earth oxide although the advantages of the invention are achievable with a sprayed coating containing at least 45% by weight, especially at least 50% by weight of the rare earth oxide. The oxides other than the rare earth oxide in the sprayed coating include Al₂O₃, Y₂O₃ and oxides of other rare earth elements.

[0017] Useful spraying techniques include flame spraying, high velocity oxy-fuel (HVOF) spraying, detonation spraying, plasma spraying, water stabilized plasma spraying, induction (RF) plasma spraying, electromagnetic acceleration plasma spraying, cold spraying, and laser spraying. The spraying technique is not particularly limited although the plasma spraying featuring a high spray output is preferred.

[0018] Depending on the operating atmosphere, the spraying is divided into atmospheric spraying and low pressure or vacuum spraying wherein spraying is effected in a chamber kept at a low pressure or vacuum. Internal pores may be reduced in order to form a more densified coating, and the low pressure spraying is recommended in this regard. However, the low pressure or vacuum spraying technique requires a low pressure or vacuum chamber in order to perform a spraying operation. This imposes spatial or time limits to the spraying operation. Then the present invention favors the atmospheric spraying technique which can be practiced without a need for a special pressure vessel.

[0019] The plasma spraying system generally includes a plasma gun, a power supply, a powder feeder, and a gas controller. The plasma output is determined by the power supplied to the plasma gun and the feed rates of argon gas, nitrogen gas, hydrogen gas, helium gas or the like. The feed rate of powder is controlled by the powder feeder.

[0020] In the plasma spraying technique, a coating is formed by operating a plasma gun to create a plasma, feeding a powder into the plasma for melting particles, and instantaneously impinging molten particles against a substrate. In order to obtain a satisfactory coating, it is requisite that spraying particles be melted fully and moved at a high flight velocity. In order that particles be melted, the residence time of particles within the plasma should be longer, which is equivalent to a lower velocity as long as a limited space is concerned, and is thus contradictory to the high velocity requirement. Increasing the input to the gun leads to increases in both the temperature and flow velocity of a plasma jet. However, the melting of particles is determined by the latent heat of fusion, particle size, specific gravity of material and gas temperature, and the flight velocity is determined by the particle size, specific gravity and jet velocity. It is then believed that the input power must be optimized for each type of powder material.

[0021] For the manufacture of a sprayed member having higher dielectric strength, with the above-described spraying conditions taken into account, it is important to use a material having a higher specific gravity as the coating. Namely, by forming a sprayed coating of an oxide having a higher specific gravity than alumina which has traditionally been used in dielectric strength sprayed members, a sprayed member having higher dielectric strength than the alumina-sprayed member is obtainable. In general, compounds of elements of greater atomic numbers often have a higher specific gravity. Of these, rare earth compounds are known to have halogen plasma resistance. However, it is unknown that such rare earth compounds have high dielectric strength. The inventor has discovered that sprayed coatings of oxides of elements having atomic number 64 to 71 have high dielectric strength as well.

[0022] Although the thickness of a sprayed coating is not critical, the preferred thickness is from 100 μm to less than 500 μm, more preferably from 100 μm to 450 μm, even more preferably from 100 μm to 400 μm. Too thin a coating may undergo breakdown due to the low dielectric strength at that thickness. Too thick a coating is liable to craze and separate from the substrate.

[0023] No particular limits are imposed to the dielectric strength (kV/mm) of the sprayed coating. The preferred dielectric strength is at least 15 kV/mm, more preferably at least 17 kV/mm as the lower limit and up to 50 kV/mm as the upper limit.

[0024] Herein, the dielectric strength can be measured according to JIS C2110, for example, using a specimen in which oxide is plasma sprayed on a metal substrate. The sprayed coating on the specimen may have a thickness of about 100 to 500 μm. Specifically, an aluminum substrate of 100 mm×100 mm×5 mm is used, one surface is blasted prior to spraying, and an oxide containing an element having atomic number 64 to 71 is plasma sprayed to form a sprayed coating of about 200 μm thick. The coated substrate is sandwiched between electrodes according to JIS C2110, and voltage is applied thereacross and increased at a rate of 200 V/sec. The voltage at which dielectric breakdown occurs is the breakdown voltage of the coating.

[0025] The voltage which is lower by 0.5 kV than the breakdown voltage is a preset voltage. If no dielectric breakdown occurs when the voltage is increased at a rate of 200 V/sec up to the preset voltage and maintained at the preset voltage for 20 seconds, that voltage is the dielectric strength (kV) of the entire sprayed coating. The thus measured dielectric strength (kV) of the entire sprayed coating is normalized as a voltage per the sprayed coating thickness of 1 mm. The normalized value is the dielectric strength (kV/mm).

EXAMPLE

[0026] Examples of the invention are given below by way of illustration and not by way of limitation.

Examples 1-7

[0027] Sprayed coatings of 200 μm thick were formed on aluminum substrates of 100 mm×100 mm×5 mm by spraying powders of oxides of atomic number 64 to 71 rare earth elements under spraying conditions: a plasma power of 35 kW, an argon gas flow rate of 40 l/min, a hydrogen gas flow rate of 5 l/min, and a powder feed rate of 20 g/min. Without sealing treatment, the sprayed coatings were subjected to a dielectric strength test.

[0028] The dielectric strength test was performed according to JIS C2110. While the voltage was increased at a rate of 200 V/sec, the voltage at which dielectric breakdown occurred was first measured. The voltage which was lower by 0.5 kV than the breakdown voltage was then assumed to be a preset voltage. If no dielectric breakdown occurred when the voltage was increased at a rate of 200 V/sec up to the preset voltage and maintained at the preset voltage for 20 seconds, that voltage was the dielectric strength (kV) of the entire sprayed coating. The thus measured dielectric strength (kV) of the entire sprayed coating was divided by the thickness (200 μm) of the sprayed coating, obtaining a dielectric strength (kV/mm). The results are shown in Table 1.

Comparative Example 1

[0029] As in Example 1, Y₂O₃ powder having an average particle size of 35 μm was sprayed, and a dielectric strength test performed.

Comparative Example 2

[0030] As in Example 1, Al₂O₃ powder having an average particle size of 30 μm was sprayed, and a dielectric strength test performed.

[0031] The results are shown in Table 1. TABLE 1 Dielectric Atomic Specific strength number Oxide gravity (kV/mm) Example 1 64 Gd₂O₃ 7.62 19 Example 2 65 Tb₂O₃ 7.81 22 Example 3 66 Dy₂O₃ 7.41 26 Example 4 67 Ho₂O₃ 8.36 19 Example 5 68 Er₂O₃ 8.65 26 Example 6 70 Yb₂O₃ 9.17 28 Example 7 71 Lu₂O₃ 9.84 25 Comparative Example 1 39 Y₂O₃ 5.03 12 Comparative Example 2 13 Al₂O₃ 3.99 10

[0032] There have been described spray coated members having a high dielectric strength. They are useful as dielectric rolls, heating substrates, electrostatic chucks and susceptors for semiconductor manufacturing apparatus and the like.

[0033] Japanese Patent Application No. 2002-379389 is incorporated herein by reference.

[0034] Reasonable modifications and variations are possible from the foregoing disclosure without departing from either the spirit or scope of the present invention as defined by the claims. 

1. A high dielectric strength member comprising a substrate and a high dielectric strength coating formed thereon in the form of a sprayed coating of an oxide containing a rare earth element having atomic number 64 to
 71. 2. The high dielectric strength member of claim 1 wherein the sprayed coating has not been subjected to sealing treatment. 